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Early diagenetic vivianite [Fe-3(PO4)(2) center dot
8H(2)O] in a contaminated freshwater sediment and
insights into zinc uptake: a mu-EXAFS, mu-XANES and
Raman study
Journal Article
http://eprints.bbk.ac.uk/1793
Version: Post-print (Refereed)
Citation:
Taylor, K.G.; Hudson-Edwards, K.; Bennett, A.J.; Vishnyakov, V. (2008)
Early diagenetic vivianite [Fe-3(PO4)(2) center dot 8H(2)O] in a
contaminated freshwater sediment and insights into zinc uptake: a
mu-EXAFS, mu-XANES and Raman study – Applied Geochemistry
23(6), pp.1623-1633
© 2008 Elsevier
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Early diagenetic vivianite (Fe3(PO4)2.8H2O) from contaminated freshwater
sediments: a µ-EXAFS, µ-XANES and µ-Raman study
Kevin G. Taylor1*, Karen A. Hudson-Edwards2, Andrew J. Bennett3 and Vladimir Vishnyakov4
1
Department of Environmental and Geographical Sciences, Manchester Metropolitan University,
Chester Street, Manchester, M1 5GD, U.K.
2
School of Earth Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, U.K.
3
4
SRS Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, U.K.
School of Biology, Chemistry and Health Sciences, Manchester Metropolitan University, Chester
Street, Manchester, M1 5GD, U.K.
* Corresponding author. E-mail: [email protected]
Abstract
The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of
organic matter, iron, zinc and nutrients as a result of past contaminant inputs. Vivianite
[Fe3(PO4)2.8H2O] has been observed to have precipitated within these sediments during early
diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100
1
µm) and micron-scale analysis techniques (SEM, electron microprobe, µ-EXAFS, µ-XANES and µRaman) have been applied in this study to obtain information upon the structure of this vivianite and
the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and
Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the Fe Kedge spectra for individual vivianite grains produces Fe-O and Fe-P co-ordination numbers and bond
lengths consistent with previous structural studies of vivianite (4 O atoms at 1.99-2.05 Å; 2 P atoms at
3.17-3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which we
interpret to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of
the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates
that Zn may be incorporated into the crystal structure of vivianite (4 O atoms at 1.97 Å; 2 P atoms at
3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination
XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an
oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This
study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other
contaminants (e.g. As) during its formation and, therefore, forms an important component of metal
cycling in contaminated sediments and waters. Furthermore, for the case of Zn, our EXAFS fits for Zn
phosphate suggest this uptake is structural and not via surface adsorption.
1. Introduction
The cycling of contaminant elements and nutrients between freshwater lake sediments and waters
plays a key role in the chemical and ecological functioning of these important environments. It has
long been known that early diagenesis plays a particularly strong role in the short- to long-term
release of the chemical species Fe, Mn, contaminant elements and P (Hamilton-Taylor et al., 1996a,
b; Bryant et al., 1997; Taylor et al., 2003). The release of P from sediments during early diagenesis
2
has been recognised to be an important control on water quality in freshwater systems (Carignan and
Flett, 1981; Jensen et al., 1992; Hupfer et al., 1995; Gonsiorczyk et al., 2001).
As diagenesis proceeds, a build up of porewater solutes may lead to mineral saturation, resulting in the
precipitation of authigenic sulphides, carbonates and phosphates. These precipitates may also sorb or
co-precipitate trace metals, thereby acting as long-term sinks for contaminants in sediments. For
example, early diagenetic sulphides in marine sediments have been shown to act as sinks for the
metallic elements Cu, Pb and Zn (Ankley et al., 1996; Parkman et al., 1996; Pirrie et al., 1999). The
absence of significant sulphate reduction in freshwaters has led to the assumption that such sinks do not
exist in freshwater sediments. It is known, however, that the iron phosphate mineral vivianite
[Fe3(PO4)2.8H2O] may precipitate during early diagenesis in freshwater sediments (Nriagu and Dell,
1974; Emerson and Widmer, 1978), and it has recently been documented that vivianite can be abundant
in nutrient-rich freshwater sediments (Dodd et al., 2000; Taylor and Boult, 2007). These sediments
result from inputs from sewage-treatment works and from fertilizer runoff from land, and are therefore
of considerable concern. Vivianite is the Fe-rich end-member of the vivianite mineral group
[X3(YO4)2·8H2O, where X=Co, Fe, Mg, Mn, Ni or Zn, and Y=P or As]. It should theoretically be
possible for vivianite to co-precipitate or sorb substantial amounts of the contaminant elements Mn, Ni,
Zn, Co and As, and the nutrient P. Since no quantitative data have been collected on the partitioning of
contaminants into vivianite during early diagenesis, nor the mechanisms by which this takes places, the
aim of this study was to test the hypothesis that naturally-occurring vivianite is a significant sink for
contaminant elements (in this case Zn) in a contaminated freshwater sediment.
2. Study Site
Vivianite analysed in this study is a diagenetic mineral present in the heavily contaminated Salford
Quays of the Manchester Ship Canal, Manchester, NW England (Fig. 1). A full description of this site,
3
and its contamination and remediation history, can be found in Taylor et al. (2003). The Manchester
Ship Canal is up to 8 m deep, steep-sided and up to 50 m wide. As a result, flow is slow, and this has
led to the accumulation of highly contaminated, organic-rich sediments. Although the site has recently
been significantly remediated as regards to contaminant inputs, the historical sediments present in the
Quays are metal-rich, containing maximum concentrations of Zn, Cu and Pb of 30,000 µg/g, 7000
µg/g, and 4000 µg/g, respectively, and Fe, Mn, Zn and P are present in porewaters at concentrations up
to 700 µM, 50 µM and 2 µM, respectively. Taylor and Boult (2007) documented the presence of Znrich glass grains in these sediments, which are undergoing diagenetic dissolution. They also
documented the precipitation of vivianite in these sediments within the first few cm of the sedimentwater interface, and suggested that this mineral may play an important role in contaminant metal
cycling at the site, but did not determine any chemical or structural data for it.
3. Experimental and Materials
Sediment cores were taken from Basin 9 of the Salford Quays (Fig. 1) using a 1 m long, 60 mm
diameter stainless steel corer with a stainless steel liner. The corer was pushed into the sediment using
lengths of steel rod, and the core retrieved. The collected core was split into 1 cm vertical sub-sections
by use of a screw-threaded plunger, in a N2-filled glove-bag to minimise oxidation of reduced species,
and samples placed into acid-washed, N2-purged centrifuge tubes and sealed. Porewater separation and
analysis of these sediments is reported in Taylor & Boult (2007).
The sediment mineralogy was characterised by whole rock X-ray diffraction (XRD) analysis of N2dried powdered samples. Samples were run on a Philips PW1050 X-ray diffractometer, using CuKα
radiation, with scans taken from 4º to 64º at a scan rate of 2º/min. Petrographic and quantitative
chemical data were obtained through the use of scanning electron microscopy (SEM) and electron
4
microprobe analysis. N2-dried samples of sediment were impregnated with epoxy resin and polished
surface blocks produced. These blocks were analysed using a JEOL 5600LV SEM, operating at 15kV
and with a working distance of 15 mm, using backscattered electron (BSE) imagery. A Link eXL
energy dispersive X-ray microanalysis system (EDX) was used to obtain semi-quantitative data on
major element compositions of mineral grains. Fully quantitative chemical data for both major and
trace elements were obtained using wavelength-dispersive X-ray analysis on a Cameca SX100 electron
microprobe. Detection limits for trace metals using this technique is on the order of 100 ppm.
X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)
spectroscopies were used to investigate how Fe and Zn are bound within the vivianites. Reviews of the
principles of these methods and mineral systems applications can be found in Calas et al. (1984) and
Brown et al. (1988). For this study, X-ray absorption spectra at the Cu and Fe K-edges and Pb LIII-edge
were collected at the minifocus XAS Station 9.2 at the Synchrotron Radiation Source (SRS) at
Daresbury Laboratory, UK. The storage ring operated at 2 GeV with a current of between 125 and
250mA. Spectra were collected using a water-cooled, double crystal Si111 monochromator detuned
70% to remove high energy harmonics. Two Kirkpatrick-Baez mirrors (substrate ULE glass coated
with 15 nm Rh on 20 nm Pt) are used to focus the beam horizontally and vertically to a spot size of ~50
µm. Stability of the focussed beam is less than the spot size. For fluorescence measurements a
Canberra 13-element solid state Ge detector with full electronics including semi-automatic windowing
was used. For both the natural and synthetic samples, encased in polished epoxy blocks, an XRF map of
the elements of interest (Fe, Zn) was collected using GDA Acquisition Version 5.6.0 software. “Hotspots”,
where concentrations of the respective element were high, were selected from the XRF map for further
analysis by XAS. Between 6 and 16 XAS scans were collected for each element for each of the
“hotspots”. Incident and transmitted intensities were measured using ionisation chambers filled with a
He/Ar mix appropriate for the X-ray energy of interest. X-ray absorption spectra were collected in
5
fluorescence mode (16 scans) for both the Fe K-edge and Zn K-edge for four vivianite grains
previously characterized by SEM and electron microprobe. X-ray absorption spectra were also
collected in transmission mode (single scan) at the Zn K-edge for a suite of Zn model compounds (Zn
metal foil, Zn3(PO4)2, ZnNO3, ZnCO3, ZnS2, ZnSO4, ZnO, Zn-sorbed goethite) to allow comparison
with experimental data for the natural samples. With the exception of Zn-sorbed goethite, the model
compounds were chemical standards of 98% purity or greater. Goethite was synthesized in the
laboratory according to the methods of Schwertmann and Cornell (1991), and Zn sorbed to it using the
methods of Parkman et al. (1999).
The X-ray absorption spectra of the Zn and Fe K-edges were measured. Data were collected from ~200
eV before to ~600 eV above the standard edge position. In the pre-edge region the X-ray energy step
size was large (~9 eV), across the edge the step size was 0.6-1 eV and in the post edge region the step
size was 1.5-2 eV. The spectra were processed using the computer programs EXCALIB, EXBROOK,
EXSPLINE and EXCURV98 compiled at the SRS (Binsted, 1998). The spectra were summed using the
EXCALIB program. This process also converted the monochromator angle into X-ray energy.
For XANES analysis the edge position was determined and the data normalised such that the edge step
was equal to one using the EXBROOK program. Linear combination fitting was performed on the
XANES portion of the data using the model compounds stated above. The proportion that each of
these standards in the spectra and a least squares residual factor were calculated. These standards were
measured for XANES and EXAFS comparisons.
For analysis of the EXAFS oscillations, the background was subtracted from the raw XAS spectra and
the data normalised using the EXSPLINE program. A spline curve, which estimates the smooth
background absorbance, was used to remove the smooth background absorbance. No more than 3
6
Comment [khe1]: Do you
mean ZnS here?
spline points were used. The data were then normalised so that the edge step is equal to one. The
energy range of the spectrum was reduced if there was significant noise present in the higher energy
region of the spectra. The EXAFS spectrum was then weighted by k3 to amplify the upper k range. The
final step was the Fourier Transform (FT) of the data in order to convert energy into a radial
distribution function over a k range of 2 to 13 Å-1.
Once the EXAFS and the FT had been calculated the data were fitted to gain structural information.
EXCURV98 (Binstead, 1998; Rehr & Albers, 1990) used exact curved wave theory (Lee & Pendry,
1975) to simulate EXAFS. The element, coordination number (CN), distance and Debeye-Waller factor
were determined using this phase and amplitude functions from this program. Standard compound
EXAFS traces were also used to compare with experimental samples in order to determine similarities
in the spectra. Single and multiple scattering were considered however multiple scattering did not
improve the fit for any sample. During fitting the Eo value, distance, Debeye-Waller factor and CN
were varied. Identification of possible elements was determined by differences in the amplitude and
phase and possible structural configurations taken from databases, e.g. Inorganic Chemical Structural
Database (www.cds.dl.ac.uk/icsd). The accuracy of the fit was determined using the R factor statistical
parameter for which a lower value indicates a better fit. The fitting procedure accurately determines
distances to within 0.02 Å, but coordination number is highly correlated to the Debeye-Waller factor
and hence larger errors (±1) exist.
For Raman microprobe spectroscopy grains of vivianite were analysed, using the same polished blocks
as for SEM and XAS, on a Renishaw 1000 Raman microscope system at Manchester Metropolitan
University. The samples were placed, and areas of analysis located, on the stage of a Leica microscope,
with 10, 20 and 50× objective lens. Measurements were made at room temperature using a 514 nm
laser argon at 10% power, and the laser spot size was less than 5 µm. Multiple (five) acquisitions were
7
made to maximize the noise-to-signal ratio. Spectra were also collected for a natural, hydrothermal
Comment [khe2]: Do we have
any more information about where
in Bolivia / which ore deposit / this
sample came from?
crystalline sample of vivianite from Bolivia.
4. Results and Discussion
4.1 Mineralogy of the vivianite grains
The presence of vivianite [Fe3(PO4)2.8H2O] was confirmed by comparing the powder XRD patterns to
the those reported for vivianite in the Mincryst mineral structural CPDS files. (Fig. 2). No quantitative
measures of vivianite abundances were made, but the presence of significant XRD peaks for vivianite
suggests that levels in the sediment are in excess of 1%.
The abundance of vivianite was confirmed by SEM observations of the bulk sediment. Backscatter
electron SEM imaging shows that the vivianites are radiating lath-shaped crystals and needle-like
masses growing within the sediment matrix (Fig. 3). Electron microprobe analyses for the vivianite
grains have been published previously (full details can be found in Taylor and Boult, 2007). The
vivianite crystals contain significant amounts of Mn (up to 5.4% MnO) and contain up to 550 µg/g of
Zn.
The classification of the grains as vivianite was also supported by Raman spectroscopy. A typical
Raman spectra for the Salford Quays vivianite grains is shown in figure 4, together with the spectra of
a well-characterised crystalline vivianite. In both cases, the bands are in good agreement. The major
band at 950 cm-1 has been assigned to the Raman active P-O stretching vibration, and bands at 1050
cm-1 and 1020 cm-1 are similar to those assigned to the P-O anti-symmetric stretching vibration (Pirrou
and Poullen, 1987; Frost et al., 2002, 2003). The bands at 230 cm-1 and 175 cm-1 are reported in
vivianite (Frost et al., 2002, 2003) and may be attributed to the Fe-O stretching vibration. The bands
8
associated with the 550 cm-1 peak are attributed to the ν4 modes, and the band at 460 cm-1, to the ν2
mode of the phosphate ion, respectively (Frost et al., 2002, 2003).
4.2. Speciation and bonding environment of Fe in vivianite grains
Nriagu (1972) has pointed out that poorly-crystalline, redox-sensitive phases such as vivianite are
prone to oxidation when exposed to standard laboratory conditions. The XRD, SEM and Raman data
presented above suggest that oxidation of the Salford Quays vivianite grains is limited. This is
confirmed by Fe K-edge XANES spectra for the vivianite grains 2 to 4 (Figure 5). However, for the
vivianite 1 grain, a shift in the Fe K-edge and details in the pre-edge area, suggests significant amounts
of Fe3+. The smaller features on the pre-edge area of grains 2 to 4, with different edge position to that in
grain 1, are consistent with an Fe2+ pre-edge feature.
The parameters derived from the EXAFS fitting for Fe are shown in Table 1, together with FeO and Fe-P co-ordination numbers and bond lengths for vivianite from Mori and Ito (1950), Fejdi et al.
(1980) and Bartl (1989). EXAFS Fe K-edge spectra and Fourier transforms for vivianites 1 and 4 are
shown in Figure 6 (the spectra for vivianites 2 and 3 are virtually identical to that of vivianite 4 and so
are not shown). The results from a first shell fitting for the apparently unoxidised vivianites (2 to 4)
suggest that Fe exists in tetrahedral coordination, with Fe-O distances of 2.03-2.05 Å. Crystallographic
studies of vivianite suggest that Fe occupies two distinct octahedral sites, with Fe2O6(H2O)4 groups
linked to two neighbouring similar groups and to four other single octahedral groups, FeO2(H2O)4, by P
(Mori and Ito, 1950; Fejdi et al., 1980; Bartl, 1989; Table 1). Because it is an averaging technique, the
EXAFS analysis is unable to resolve these differences in our vivianites, but the bond distance of 2.032.05 Å is similar to the average first shell Fe-O distances calculated for the data of Fejdi et al. (1980)
and Bartl (1989; Table 1) and the average second shell distance of 2.02 Å calculated for the data of
Mori and Ito (1950). The tetrahedral coordination fit for our vivianites also matches the second shell of
these previous studies, suggesting that either our data are not of sufficient quality to fit the additional
9
shell of 2 O around the Fe, or that the Fe is more disordered within the normally octahedral site. The
identification of a shell of two P atoms at a distance of 3.23 Å in grains 2 to 4 is typical of the vivianite
structure (Mori and Ito, 1950; Fejdi et al., 1980; Bartl, 1989; Table 1).
First and second shell fits for the apparently oxidized vivianite 1 grain are similar to those of
vivianites 2 to 4 in terms of the scatterer atoms and their co-ordination, but bond distances are slightly
shorter (O – 1.99 Å; P – 3.17 Å; Table 1). This is likely due to the formation of an FeIII phase.
4.3. Bonding environment of Zn in vivianite grains
The Zn K-Edge XANES data for the vivianite grains, along with those for model compounds, are
shown in Figure 7. Parameters from the Zn EXAFS fitting for vivianites 2 and 3 (the only two grains
with clean enough spectra for fitting) are shown in Table 1, and the K-edge EXAFS spectra and Phase
Shifted Fourier transforms in Figure 8.
XANES linear combination fitting for the apparently unoxidised (or very minimally oxidised)
grains 2 to 4 suggest 85% phosphate or Zn-sorbed goethite (the XANES for these are almost identical)
and 15% Zn sulphate (errors in linear combination fitting are +/- 10%). EXAFS fits for grain 2 define
three shells of atoms around the Zn: a first shell of 4 O at 1.97 Å, a second shell of 2 O at 2.98 Å, and a
third shell of 2 O at 3.17 Å. The first and third shells exactly match O and P distances and coordination
numbers reported for Zn phosphate dehydrate by Sarrett et al. (2001). This, together with the XANES
fitting, suggests that Zn may be substituting for Fe in the vivianite structure. The shorter first shell O
distance for Zn compared to Fe (Table 1) may be due to the larger ionic radius of the former.
The first shell of 4 O atoms fit for Zn also matches that of a first shell fitting of Zn sorbed on
goethite (Trivedi et al., 2001). This, together with the XANES fitting and the small amount of FeIII in
grain 2 (see above), suggest that some Zn-sorbed goethite may be present. However, Trivedi et al.
(2001) also report a second shell of 1.74 to 2.89 Fe atoms with Zn-Fe distances of 3.51-3.54 Å, which
10
do not match any of our calculated distances (Table 1), suggesting that, if present, the amounts of Znsorbed goethite in our samples are small.
The second shell of O atoms at 2.98 Å fit for grain 2 is unlike any previously reported distances
for metal-O bonds in vivianites, or for Zn sorbed to goethite (Table 1; Trivedi et al., 2001). Given the
XANES fitting, it is possible that this second shell fit is indicative of Zn sulphate either within, or
sorbed on, the vivianite structure. Sulphate is known to substitute for phosphate in other minerals (e.g.,
orpheite PbAl3(PO4,SO4)2(OH)6), and moderate concentrations of SO42- are reported in Salford Quays
porewaters (Taylor and Boult, 2007). If true, then the amounts of Zn sulphate substituting in the
structure are probably very low, given that no bands attributable to sulphate (ν1 983, ν2 450, ν3 1105
and ν4 611 cm-1; Ross, 1974) are recorded in the Raman spectra (Figure 4). The Zn sulphate may
therefore be forming post-vivianite formation, possibly during oxidation of the grain. Despite all of
this, the bond distance of 2.98 Å does not match any Zn-O distances reported for the Zn sulphate
mineral zincosite (2.312, 2.113, 1.970 Å; Wildner and Giester, 1988). It is therefore possible that the
2.98 Å bond distance represents an average of different Zn bond types including sulphate or Zn sorbed
to goethite, as above. (Figure 5).
For grain 3, only one shell of 4 O at 1.97 Å was fit around Zn. This is identical to the first shell
fit for grain 2, but in this case the lack of extra shells suggests less long-range order (due to either lower
concentration of Zn or fewer scans collected) and possibly sorption of Zn on the vivianite surface rather
than incorporation within the structure. As with grain 2, the fit of 4 O at 1.97 Å also matches that of
Zn-sorbed goethite (Trivedi et al., 2001), and this, together with the possible presence of FeIII, suggests
that Zn-sorbed goethite may also be present.
For the apparently oxidised grain 1, linear combination XANES fitting yielded 42% Zn goethite
or phosphate, and 58% Zn sulphate. This may be indicative of secondary Zn sulphate or Zn-sorbed
11
goethite formation following oxidation of this grain. As with grain 2, this may indicate the presence of
structural Zn phosphate and sulphate, as well as secondary Zn sulphate and Zn-sorbed goethite.
5. Implications
The study has shown that the use of µ-Raman and µ-XAS analysis of low temperature mineral
precipitates in sediment systems can provide critical information upon the structure and composition of
these minerals. Given that low temperature diagenetic minerals are commonly fine-grained and poorly
crystalline, the application of these techniques promises to provide new insights into the mechanisms of
precipitation and nature of these important minerals. We have shown for the first time that Zn may be
incorporated into the crystal structure of vivianite. These findings indicate that vivianite can indeed act
as a significant sink for trace metals in contaminated sediments during early diagenesis. Early
diagenetic sulphide mineral precipitates have been documented previously to act as sinks for trace
metals in marine and brackish sediments (Parkman et al., 1996; Pirrie et al., 1999; Morse and Luther,
1999) and these minerals are routinely considered in contaminated marine sediment assessment
(Ankley et al., 1996). Our study is the first to consider the incorporation of the potentially toxic
element Zn in vivianite, an important mineral in anoxic environments such as acid mine drainage sites
(Ueshima et al., 2004), rivers (House, 2003), lakes (Fagel et al., 2005) and urban water bodies (Taylor
et al., 2003). There is a need to study the role of vivianite in the uptake of other potentially toxic
elements in freshwater systems (e.g. As, Pb, Sb).
6. Conclusions
12
1) Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis
confirms the presence of vivianite [Fe3(PO4)2.8H2O], precipitated during early diagenesis, within the
sediments of the urban water body of the Salford Quays, Greater Manchester, NW England.
2) EXAFS model fitting of the Fe K-edge spectra for individual vivianite grains produces Fe-O and FeP co-ordination numbers and bond lengths consistent with previous structural studies of vivianite. One
analysed grains displays evidence for the presence of a significant Fe3+ component, which we interpret
to have resulted from oxidation during sample handling and/or analysis.
3) EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model
compounds, indicates that Zn may be incorporated into the vivianite as a Zn-phosphate structure. Low
levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting
and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the
inclusion of Zn sulphate into the vivianite grains during precipitation.
4) This study confirms that early diagenetic vivianite may act as a sink for contaminant metals during
its formation and, therefore, forms an important component of metal cycling in contaminated sediments
and waters. Furthermore, for the case of Zn, our data suggest that this uptake is structural and not
surface adsorption. This observation highlights the need to consider the role of vivianite in the uptake
of other potentially toxic elements in freshwater systems (e.g. As).
Acknowledgements
This research was funded by the UK Natural Environment Research Council (Grant GST/02/2255) and
the UK CCLRC Daresbury Laboratory (provision of beamtime through direct access award 45/241).
David Plant, University of Manchester, is thanked for help with the electron microprobe analysis and
13
for providing the crystalline vivianite sample. We would like to thank Natalie Fagel for constructive
comments on an earlier version of the manuscript.
14
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18
Figure Captions
Figure 1 – Location map of the Salford Quays and the Manchester Ship Canal, showing the location of
the sediment core samples.
Figure 2 – A typical X-ray diffractogram of a sample of sediment from the Salford Quays (this sample
from 10cm below the sediment surface). The vivianite peaks are marked (V), with the calculated dspacings (in angstroms). Other mineral peaks are marked (Q = quartz; M = mica; K = kaolinite; C =
calcite).
Figure 3 – Backscatter electron images of the vivianite grains analysed by Raman and XAS in this
study. The black areas surrounding the grains are resin.
Figure 4 - Raman spectra of one of the vivianite grains (vivianite 2), together with the spectra for a
crystalline vivianite sample of hydrothermal origin, Bolivia.
Figure 5 – Normalised Fe XANES for vivianite grains 1 to 4.
Figure 6 –Normalised Fe K-edge EXAFS and Phase Shifted Fourier Transform spectra for vivianites 1 to
4. Dashed lines are least-squares fits using parameters shown in Table 1.
Figure 7 – Normalised Zn XANES for vivianite grains 1 to 4, and for model compounds.
Figure 8 – Normalised Zn K-edge EXAFS and Phase Shifted Fourier Transform spectra for vivianites 2
and 3. Dashed lines are least-squares fits using parameters shown in Table 1.
19
N
SALFORD
MANCHESTER
Basin 9
Salford
Quays
l
el
Manchester
Ship Canal
er
Irw
v
Ri
0
m
500
Q
Q
Counts (ln)
200
Q
C
100
M
4
8
V
M
KV
12
16
20
V
MK
F
24
28
Degrees 2Φ
Q
M
32
36
Q
Q
40
C
Q
Q
44
48
52
QQ
56
60
64
A
B
vivianite
C
vivianite
vivianite
Raman
Intensity
950
190
225
460
425
580
1050
1020
1610
Hydrothermal
vivianite
1045
175
100
230
460
1590
580
500
1000
Wavenumber /cm-1
1500
Salford Quays
vivianite grain 2
2000
Vivianite 4
Vivianite 3
Vivianite 2
Vivianite 1
7100
7150
7200
Vivianite 1
6
Vivianite 4
EXAFS (experiment)*k**3
CW SS (theory)*k3
6
3
k3 chi(k)
k3 chi(k)
3
0
-3
0
-3
-6
-6
3
25
EXAFS (experiment)*k**3
CW SS (theory)*k3
5
7
9
3
FT (experiment)
FT (theory)
5
7
9
FT (experiment)
FT (theory)
20
FT k3 chi (k)
FT k3 chi (k)
20
15
10
5
16
12
8
4
0
1
3
5
7
r/Angstroms
9
0
1
3
5
7
r/Angstroms
9
Zn-sorbed on goethite
Zn phosphate
Zn sulphate
Zn metal
Zn nitrate
Zn oxide
Vivianite 1
Vivianite 2
Vivianite 3
Vivianite 4
9650
9700
9750
Vivianite 2
Vivianite 3
EXAFS (experiment)*k**3
CW SS (theory)*k3
6
k3 chi(k)
k3 chi(k)
3
0
-3
-6
3
25
4
5
6
7
8
9
FT (experiment)
FT (theory)
FT (experiment)
FT (theory)
FT k3 chi (k)
FT k3 chi (k)
20
15
10
5
0
1
3
5
7
r/Angstroms
9
Table 1. Parameters used in fitting Fe and Zn K-edge EXAFS data for Salford Quays vivianite grains.
2σ2 is the Debye-Waller type factor, and the R factor indicates quality of fit (lower R factor indicates a
better fit). Structural data of vivianite (averages) from Mori and Ito (1950), Fejdi et al. (1980) and
Bartl (1989) are shown for comparison purposes. Errors in this table are as follows: Number of atoms ±
1; r ± 0.02Å; DW ± 0.002Å2.
Element
Analysed
Grain
Scatterer
No of Atoms
R (Å)
2σ2 (Å2)
R factor
Fe
1
O
P
O
P
O
P
O
P
4
2
4
2
4
2
4
2
1.99
3.17
2.03
3.25
2.05
3.21
2.05
3.23
0.031
0.007
0.025
0.020
0.023
0.012
0.028
0.027
39.73
O
O
P
O
Not determined
4
2
2
4
Not determined
1.97
2.98
3.17
1.97
0.025
0.021
0.028
0.021
41.70
O
O
P
2
4
2
1.98
2.02
3.28
O
O
P
2
4
2
2.06
2.18
3.22
O
O
P
2
4
2
2.06
2.19
3.23
2
3
4
Zn
1
2
3
4
Fe
Fe
Fe
Mori and
Ito, 1950
Fejdi et
al., 1980
Bartl,
1989
20
39.01
48.80
39.48
31.90